Do you see what I see?
Lesson 3: Is seeing believing?
June 2012
The Electromagnetic (EM) and
Human Visible Spectra
Source: http://www.antonineeducation.co.uk/physics_gcse/Unit_1/Topic_5/topic_5_what_are_the_uses_and_ha.htm
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Do you see what I see?
Lesson 3: Is seeing believing?
June 2012
The Additive (RGB) Color Wheel
Source: http://www.d.umn.edu/~mharvey/th1501color.html
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Do you see what I see?
Lesson 3: Is seeing believing?
June 2012
Teacher Content Background
Electromagnetic radiation can be thought of as consisting of waves, and therefore every kind of EM
radiation has an associated wavelength, the physical distance between two successive peaks in that
wave. The full EM spectrum is shown above.
Human vision, however, is limited to what we call visible light, or EM radiation found within the visible
spectrum, and that is the focus of the lesson. The “ROYGBIV” (red-orange-yellow-green-blue-indigoviolet) mnemonic can be useful in memorizing the order, by wavelength, of the colors that comprise the
spectrum. Colors at the violet end of the visible spectrum have shorter wavelengths (and higher
energy), while the red end of the spectrum has longer wavelengths (and lower energy). Immediately
past the red end of the visible spectrum is the infrared region, which is beyond human perception but is
often associated with heat. Immediately beyond the purple end of the visible spectrum is the ultraviolet
region, also beyond human perception but readily apparent to anyone who has ever had a sunburn! The
color system invoked to explain human visual perception is referred to as the RGB (“red-green-blue”)
model or the additive color model.
It is the same model that is used to generate colors on computer screens and other electronic displays.
Because of how the human retina operates, blue, green and red can be thought of as “primary
physiological colors,” as shown above in the additive color wheel. When we perceive the color red, for
example, it is because red light, whether emitted from a light bulb or reflected from an object, is
exclusively activating our “red” cones, whereas green and blue result from the exclusive activation of
our “green” and “blue” cones, respectively. By contrast, yellow, cyan and magenta can be thought of as
“secondary physiological colors”; each results from the simultaneous and equal activation of two cone
cells. For example, a yellow chair appears yellow under full-spectrum (white) lighting because both red
and green wavelengths of light are being reflected from the chair into the eye, activating the “red” and
“green” cone cells equally and simultaneously; reference the additive color wheel to reinforce this. If all
three cone varieties are activated equally and simultaneously, the resulting perception is white (also
shown on the color wheel).
The “complementary physiological color pairs” are blue & yellow, red & cyan, and green & magenta
(colors across from each other on the additive color wheel). An object of color X best absorbs light at
the wavelength of its complementary color. For example, a yellow chair best absorbs light in the blue
range of the spectrum. When two or three of the different cone varieties are activated unequally, you
perceive non-primary or non-secondary physiological colors, such as orange, brown or purple. The
“Online Color Scheme Generator” (http://www.colorschemer.com/online.html) provides a good analogy
for demonstration this principle. The “R” (red), “G” (green), and “B” (blue) inputs on the left margin of
the webpage may each be set anywhere from 0 to 255. After pressing “Set RGB,” the square in the top
left corner will change depending on how each of the three primary colors has been weighted. For
example, a setting of “R: 0, G: 0, B: 255” will yield “pure” blue, while “R: 255, G: 0, B: 255” will yield
“pure” magenta.
Finally, it is important to understand the distinction between the optical phenomena reflection,
absorption, transmission and emission. Imagine bringing an apple into the classroom. Assuming the
classroom is using white (full-spectrum) lighting, these overhead lamps are emitting light at nearly all
the wavelengths in the visible spectrum; emission is the process of generating light (or other EM
radiation). Some of the light emitted by the overhead lamps will hit the apple. A part of this light will be
absorbed by the apple. Absorption is process by which light energy is captured by some form of matter.
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Do you see what I see?
Lesson 3: Is seeing believing?
June 2012
By contrast, another part of the emitted light in the room will be reflected by the apple; if the apple is
red, then the red wavelengths of the room light will be best reflected by the apple. Reflection is the
process by which light contacts and is redirected by some form of matter. Some of the light reflected off
of the apple is furthermore absorbed by our retinas, activating our “red” cone cells and thus creating the
perception that the apple is red. Finally, in order for the light to travel from the overhead lamps to the
apple and then from the apple to our eyes, it had to have been transmitted through the air.
Transmission is the transit of light through some non-opaque medium; transmission can occur through
vacuums or solids, liquids and gases that don’t completely absorb or reflect light.
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